Juvenile common marmosets with no prior exposure to neonates were tested at varying intervals up to 4 weeks after the birth of neonates into their family group. Retrieval latencies averaged around 40 seconds to both familiar and unfamiliar infants in the same juveniles who had completely failed to show retrieval behavior when tested with unfamiliar infants prior to this experience. As little as 60 hours of exposure to neonates was sufficient for retrieval behavior to be expressed. Thus, the data from this study strongly indicate that caregiving is an active process promoted by stimuli arising from the presence of a neonate in the family group. The stimulus role for the neonate appears to be as important in adults as in juveniles and subadults. Study of vocal development in rhesus macaques indicated that differential rearing (nursery with peers vs mother with mixed age group members) may influence the rate of maturation of vocal signalling, and that, in mother-reared infants, males and females develop at different rates with respect to some aspects of vocal behavior.